1. Field of the Invention
The present invention relates to a sheet heater and an image fixing device including the sheet heater.
2. Description of Related Art
Image forming apparatus such as copiers and laser beam printers have a built-in image fixing device. The image fixing device includes a heater that is allowed to come into pressure-contact with an unfixed image to form a fixed image (see, for example, Japanese Patent Application Laid-Open Nos. 2006-294604, 2009-92785, and 2009-109987). The heater is provided as a heat fixing belt as illustrated in FIGS. 5A and 5B, for example. Heat fixing belt 10 in FIGS. 5A and 5B is a pipe-shaped member, and includes first insulation layer 1, specific resistance heater layer 2, second insulation layer 3, releasing layer 4, and electrode layer 5. FIG. 5A is a sectional view of heat fixing belt 10 taken along the axial direction of the pipe, and FIG. 5B is a sectional view taken along the line II-II in FIG. 5A.
Specific resistance heater layer 2 of heat fixing belt 10 includes a polyimide resin as a matrix resin, and conductive materials such as fine metal particles and carbon materials. In addition, electrode layer 5 is formed by applying, for example, a conductive paste containing a polyimide, resin as a matrix resin on specific resistance heater layer 2.
As described above, it has been known in the art to employ a conductive paste to form an electrode layer of a heat fixing belt. However, the electrode layer formed of a conductive paste occasionally has low mechanical strength. In addition, a voltage of 100 to 240 V is typically applied to the electrode layer, causing the temperature of the heat fixing belt to rise above 200° C. When the supply of electricity is stopped, the heat fixing belt is cooled down to normal temperature in several seconds. The heat fixing belt undergoes such harsh environmental changes. Thus, cracks and may be formed as a result of repetitive use and defect holes may be formed t as a result of sparks in the heat fixing belt.
To counter the foregoing drawback, it is conceivable to adopt a metal plate as the electrode layer of the heat fixing belt. With such a configuration, however, the electrode layer formed of a metal plate and a specific resistance heater layer are occasionally separated from each other when the belt is repeatedly used.